Cathodoluminescent light source

ABSTRACT

A cathodoluminescent light source comprising a field-emission cathode serving as a source of electrons, an anode having a specular light-reflecting surface, and an electron-excited phosphor applied to the specular light-reflecting anode surface. The cathode and anode are enclosed in an evacuated housing having a transparent surface, so as to let the electron-excited phosphor on the anode surface, be irradiated with an electron beam, and let the luminous flux resulting from the process of cathodoluminescence, to emerge.

TECHNICAL FIELD

The present invention relates of sources of optical radiation used forlighting and/or forming images using displays of diverse constructionsand purposes.

BACKGROUND ART

A variety of light sources are made use of virtually in every field ofhuman activities. In an overwhelming majority of instances the operatingprinciple of light sources implies electric current conversion intolight. Depending on their specific use light sources should meetdefinite requirements as to radiation intensity and directivity, itsspectral distribution, overall dimensions, and other characteristics.The most important parameter of any light source is the efficiency ofelectric energy conversion into light. Hence the parameters of thevarious light sources may vary within broad ranges depending on physicalfundamentals used for light emission. In particular, said efficiency ofelectric energy conversion into visible light in incandescent lamps isas low as 15%. The efficiency of electric energy conversion into lightsources based on electroluminescence of various kinds depends badly onthe wavelength of the light emitted and varies from 0.015% for ashort-wave (blue) spectral range to 15% for a long-wave (red andinfrared radiation. In various gas-discharge light-emitting apparatusand devices the energy conversion efficiency varies from 1 to 20%depending on the kind of discharge and spectral characteristics of theradiation. Gas-discharge light sources are utilized in particular as UVradiation sources for further emission of visible light due tophotoluminescence. Efficiency of conversion of UV radiation energy intovisible one is as high as 60% which brings an energy efficiency (i.e., atotal efficiency of electric energy conversion into visible light) inphotoluminescent lamps to as high level as 10%. Despite a relativelyhigh energy efficiency of photoluminescent lamps they suffer from anumber of disadvantages. One of the most substantial disadvantages isuse of mercury therein. There may be used electron beams instead of UVradiation for exciting luminescence. In such a cathodoluminescentprocess the efficiency of conversion of UV radiation energy into visiblelight may reach 35-40%. Besides, a total efficiency ofcathodoluminescent light sources is the function of the amount of powerconsumed for establishing a required electron beam.

Serving as exemplary cathodoluminescent light sources may be variouscathodoluminescent lamps, indicators, TV tubes, vacuum luminescentdevices, and the like. As a rule, an electron beam in such devices isestablished due to thermionic emission from a high-temperature cathodecf., e.g., British patent #2,009,492 and RF patent #2,089,0070).Efficiency of electric energy conversion into visible light in suchdevices is but too low on account of the fact that a considerableproportion of the energy must be spent for heating the cathode.Furthermore, the fields of application of such devices are badlyrestricted by complicated production process thereof, as well as overalldimensions and requirements imposed upon operating conditions of saiddevices. On the other hand use of other kinds of stimulated emission ofelectrons as a source thereof (such as photo-emission, secondaryelectron emission, and the like likewise fails to providehigh-efficiency electric energy conversion into light.

An alternative method for producing an electron beam resides in use ofthe effect of field (or spontaneous) emission. Unlike the thermionic,photoelectronic, and other kinds of stimulated emission the fieldemission of electrons occurs without energy absorption in the materialof the cathode (emitter) which establishes a prerequisite for provisionof high-efficiency light sources. However, provision of electron beamsusing field-emission cathodes and having a current density high enoughfor practical use involves a very high electric field intensity(potential gradient) effective on the cathode surface (10⁸-10⁹ V/m).Such a high field intensity requires in turn the use of adequately highvoltage values and/or of cathodes shaped as thin spires or blades thatcontribute to a local electric field amplification. Accordingly, voltagevalues accessible from practical standpoint involve provision of spiresand blades of micron and sub-micron range which adds substantially tothe cost of their production. Moreover, electron emission occurs to beextremely unstable due to high sensitivity of such micron-size spirestructures to environmental conditions. Said circumstances impedesubstantially use of spire- and blade-type field-emission cathodes inbroad-purpose apparatus and devices.

Known in the art presently is a cathodoluminescent light source whereina fine thread of an electrically conductive material is made use of as afield-emission cathode (cf. WO97/07531). In a lamp of this type thecathode is enclosed in an evacuated glass bulb whose inside surface hasa transparent electrically conductive coating serving as an anode. Alayer of a phosphor capable of light emission under the effect of anelectron stream is applied to said electrically conductive coating.However, one of the disadvantages inherent in such a constructionresides in that in order to provide an adequately high electric fieldintensity required for electron emission and the values of a voltagebetween the anode and cathode acceptable for practical use, one isforced to utilize threads having extremely small diameter (from 1μ to15μ). Too a low mechanical strength of such fine threads presentsconsiderable problems in making cathodes for the light sources underconsideration. One more disadvantage of said construction ofcathodoluminescent lamps lies with the fact that an electron beamperforms a most efficient excitation on that side of theelectron-excited phosphor layer which faces the cathode, that is,inwards the glass bulb. Hence a considerable proportion of the luminousflux is absorbed in those electron-excited phosphor layers which arelocated nearer to the transparent outside bulb surface. Light absorptionresults in a loss of a part of energy and an affected general efficiencyof lamps of a given type.

Known in the art are carbon materials, wherein field emission isobserved to occur at a much lower electric field intensity (10⁶-10⁷ V/m)which is due to nanometer dimensions of the structural elements thereof,as well as due to specific electronic properties inherent innanostructurized carbon (cf. WO 00/40508 A1). Use of such materials aselectron emitters (cathodes) enables one to substantially reduce thevalue of a voltage applied between the anode and cathode to produce anelectron beam.

One more cathodoluminescent light source is known to appear as acylinder-shaped thermionic diode with a field-emission cathode appearingas a dia. 1 mm metal wire provided with carbon nanometer-size tubes(nanotubes) applied to the wire surface (cf. J.-M. Bonard, T. Stoeckli,O. Noury, A. Chatelain, App. Phys. Lett. 78, 2001, 2775-2777). Use ofcarbon nanotubes makes it possible in this case to reduce the voltagevalues used in the device. However, one of the disadvantages the lampsof said type suffer from is the use of carbon nanotubes whose productionprocess involves utilization of a metallic catalyst. The nanotubesmanufactured by such a process carry metal particles at the end thereof,whereby the tubes want further chemical treatment to remove saidparticles and attain a required electrode emission efficiency. Anotherdisadvantage inherent in said lamps is the fact that subjected toelectron excitation is also an electron-excited phosphor disposed on aninside surface of the cylinder-shaped glass bulb. Part of the lightemitted by said layer is absorbed when the light passes towards thetransparent lamp surface, thereby affecting adversely a total efficiencyof electric energy conversion into light.

DISCLOSURE OF THE INVENTION

It is a principal object of the present invention to provide acathodoluminescent light source capable of ensuring as high electricenergy conversion into light as possible.

Other objects of the invention are a simplified construction andproduction process techniques of the lamp proposed herein.

Said objects are accomplished by the present invention due to firstly,the fact that the anode surface facing the cathode has a specular lightreflecting surface.

In addition, said objects are accomplished also due to a specialconstruction arrangement of the light source used.

In one of the preferred embodiments of the invention the housing of alight source is cylinder-shaped, the specular anode surface overlapspart of the inside surface thereof, whereas the remainder surface of thehousing is transparent to the light arising thereinside to pass through.The cathode is shaped as a thread arranged along the longitudinal axisof the housing.

In another preferred embodiment of the present invention the housing isspherical-shaped, the specular anode surface overlaps part of the insidesurface of said sphere, and the cathode is shaped as a spire located atthe center of the spherical surface of the housing or nearby saidcenter.

In one more preferred embodiment of the present invention the lightsource is provided with a base enclosed in a transparent housing adaptedto be evacuated and provided with either grooves or hemisphericalrecesses, the surface of both said grooves and recesses being a specularlight reflecting one and the grooves and recesses themselves perform thefunctions of an anode, whereas the cathodes appear either as threadslocated above said grooves along them, or as spires situated over thecenters of the hemispherical recesses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of an embodiment of a cylinder-shaped lamp, accordingto the invention (side view (1 a), end view (1 b) and perspective view(1 c);

FIG. 2 is a view of an embodiment of a spherical lamp, according to theinvention;

FIG. 3 is a view of an embodiment of a flat lamp, according to theinvention, comprising a number of cathodes and anodes, wherein 3 a and 3b show a perspective view and a plan view, respectively, of a lamp withthreadlike cathodes, and 3 c and 3 d show those of a lamp withspire-shaped cathodes;

FIG. 4 is same enclosed in a housing;

FIG. 5 represents volt-ampere characteristics of a cylinder-shaped lampmade according to the present invention; and

FIG. 6 represents a relationship of luminance vs voltage for a lamp madeaccording to the present invention.

EMBODIMENTS

A cathodoluminescent lamp according to the invention may be shaped as acylinder-shaped thermionic diode schematically shown in FIG. 1. To thisend, first a cylinder-shaped glass bulb 1 is prepared, whereupon a layer2 of aluminum or some other metal featuring good light-reflectingproperties is applied to a portion of the inside cylinder-shaped bulbsurface. Said reflecting metal layer is electrically connected to anelectrode brought to the outside surface of a bulb 3. A layer 4 of anelectron-excited phosphor is applied to said reflecting metal layer 2.The bulb 3 accommodates a field-emission cathode appearing as acylinder-shaped metal wire 5 coated with a layer of a carbon material 6featuring high-efficiency field electron emission. It is expedient touse as said carbon material a film consisting of a nanometric-sizegraphite crystallites and carbon nanotubes as taught in WO00/40508 A1.The cathode is reasonable to be arranged lengthwise the bulblongitudinal axis and is electrically connected to an electrode whichbrought to the outside surface 7 of the bulb 3. The diameter of the wirethe cathode is made from and that of the cylinder-shaped bulb 3 are soselect as to provide, with the preset operating voltage values appliedacross the anode and cathode, such a level of electric field intensityeffective on the cathode surface that is required for establishing anelectron emission current of a required magnitude. For instance, for theaforementioned carbon material, as taught by WO00/40508 A1, a requiredfield intensity (F) equal to or in excess of 1.25r10⁶ V/m may beattained at a voltage (V) equal to or in excess of 4 kV applied acrossthe cathode having a diameter d=1 mm and the anode having a diameterD=20 mm in accordance with a known formula F=V/[dln(D/d)]. Accordingly,when applying a voltage in excess of 4 kV the electron emitted from thecathode are accelerated in the interelectrode space to make theelectron-excited phosphor applied to the anode surface, glow. It is dueto the provision of a specular reflecting anode surface that a luminousflux 8 of cathodoluminescence is directed towards a transparent(non-metallized) area of the surface of a glass bulb 9. The lamp may usefurther electrodes (not shown) aimed at control over the electron beam(that is, focusing, deflection, modulation). Once all electrodes havebeen fixed in position inside the lamp, the latter is evacuated to arequired level and hermetically sealed. To maintain a required vacuumlevel in the lamp for a prolonged period of time use can be made of agetter.

The cathodoluminescent lamp according to the invention may appear as aspherical thermionic diode shown schematically in FIG. 2. Such being thecase the lamp is made from a spherical-shaped glass bulb 10. Part of thearea of the inside bulb surface is provided with s metallic coating 11serving as the anode. The anode surface is coated with anelectron-excited phosphor layer 12. The cathode appears as a spirehaving a surface 13 close to a spherical one. The cathode surface iscoated with a carbon film 14 similar to that mentioned in the precedingexample. A spherical cathode portion coated with the carbon film islocated at a point disposed substantially at the bulb center. Thecathode and anode are electrically connected to the electrodes broughtto the outside surface of the glass bulb 15 and 16. Like in thepreceding example a luminous flux resulting from cathodoluminescenceemerges from the lamp through a portion of its surface remainingnon-metallized. In the case of a spherical lamp a formula associatingthe lamp geometrical characteristics (i.e., cathode diameter d and anodediameter D) with applied voltage (V) and electric field intensityappears as F=2VD/[d(D−d)]. According to said formula, the sphericalconfiguration enables a required field intensity to be attained on thecathode surface when using lower field intensity values, or with smalleroverall dimensions of the lamp electrodes compared with a cylindricalconfiguration.

The cathodoluminescent lamp according to the invention may also appearas a flat device having a number of cathodes and anodes. FIG. 3illustrates schematically a light-emitting structural component of aflat lamp, comprising cathodes and anodes. In such a case the lamp anodemay appear as a plate 18 having one or more recesses having eithercylinder-shaped profile 19 or spherical-shaped profile 20. Said platemay be made from an electrically conductive light-reflecting material orfrom an insulant (e.g., glass) and is then metallized. The metallizationlayer may be either a continuous one 21 or appear as separateelectrically insulated portions 22. The light-reflecting anode surfaceis coated with a layer of electron-excited phosphor, whereas thecathode, like in the preceding embodiments, appears as electricallyconductive threads 23 or spires 24 coated with a carbon layer whichprovides for the required electron emission characteristics. Saidthreads are situated above the anode plate surface so as to causecatodoluminescence under the effect of emitted electrons. Glass orquartz fibers 25 may be made use of for mechanically securing at apreset spacing from the anode. Cathode threads and threads withspire-shaped cathodes are put onto said fibers perpendicularlytherewith. Said emitting and insulating threads may be prefastentogether to form a single network. The latter being the case, such anetwork from the cathodic and insulating threads is placed onto theanodic plate to form a diode configuration.

Once the thread-like cathode has been mechanically held with respect tothe anodic plate, the entire structure in an assembled state is enclosedinto a hermetically sealed housing having a transparent surface forlight to pass through. FIG. 4 shows schematically a flat lamp comprisinga light-emitting element provided with anodes 26 and cathodes 27, aswell as with dielectric fibers 28 isolating said anodes and cathodesfrom one another. A hermetically sealed lamp housing 29 compriseselectric leads for connecting cathodes 30, anodes 31, and otherelectrodes, as well as has a transparent window for a luminous flux 32to emerge.

FIG. 5 presents volt-ampere characteristics of a cylinder-shaped lampmade according to the present invention. The lamp cathode in this caseis made from dia. 1 mm nickel wire coated with a layer of a carbonlight-emitting material, the cathode length is 40 mm. The anode appearsas a metallized surface of the inner side of a dia. 20 mm glass bulb;the metallized area is 20 mm wide and 40 mm long. Said volt-amperecharacteristics are presented as characteristic curve illustratingamperage (I) vs voltage (V) (FIG. 5 a) and in the Fowler-Nordheimcoordinates (that is, logarithm of the ratio of I/V² from I/V) (FIG. 5b). In the latter case the relationship has a linear character typicalof field electron emission.

FIG. 6 displays a relationship of lamp luminance (B) vs voltage (V)applied across the anode and cathode. Said relationship refers to thecase of a lamp using an electron-excited phosphor having chemicalcomposition of Gd₂O₂S:Tb (available from NICHIA Corp.).

Practical evaluation carried out against the data presented in FIGS. 5and 6 demonstrates that the lamps made according to the presentinvention feature the efficiency of electric energy conversion intolight as high as 30% which exceeds much the efficiency of all lightsources known up-to-date.

Industrial Applicability

The cathodoluminescent light source proposed in the present invention isa novel type of light-emitting devices (lamps). Construction of lampsmade in accordance with the present invention enables one to attain muchhigher efficiency of electric energy conversion into light compared withother known types of light sources. Lamps of the given type can findapplication for diverse purposes to substitute heretofore-known lightsources. Lamps of the given type offer substantial advantages overheretofore-known light sources whenever high illuminance is requiredwith a minimum heat release. Neither construction of the lamps underconsideration nor production process techniques thereof involve use ofnoxious or ecologically harmful materials. By appropriately selectedelectron-excited phosphor the lamps of the given type may produce lighthaving preset spectral characteristics alongside with high energyefficiency. Lamps of herein-proposed construction can find use inliquid-crystal displays and indicators to provide lower powerconsumption and adequate luminosity. And finally, lamps in questionhaving electrically insulated anodes may serve as displays, indicators,and similar apparatus for presenting visual information.

1. A light source, wherein it comprises: a housing adapted to beevacuated, wherein at least part of the surface area thereof istransparent, said housing accommodating: at least one anode whosesurface facing the cathode is adapted to perform specular lightreflection and is coated with a layer of electron-excited phosphor; andat least one cathode producing an electron beam as result of fieldemission.
 2. The light source of claim 1, wherein the housing iscylinder-shaped, the cathode is filiform and is arranged substantiallyalong the longitudinal housing axis, the specular reflecting anodesurface overlaps partially the inside cylinder-shaped housing surface,while the remainder portion of the surface thereof is transparent to thelight generated inside the housing.
 3. The light source of claim 1,wherein the housing is spherical-shaped, the cathode is spire-shaped andis arranged substantially at the center of the spherical housing,specular reflecting anode surface overlaps partially the insidespherical-shaped housing surface, while the remainder portion of thesurface thereof is transparent to the light generated inside thehousing.
 4. The light source of claim 1, wherein the anode surface isformed by applying an electrically conductive coating to a portion ofthe inside housing surface.
 5. The light source of claim 1, wherein itis provided with a number of anodes having a shape approximating to asemi-cylindrical one and located on a substantially planar base or madetherein, and the cathodes are thread-like, said threads being disposedabove and along said anodes.
 6. The light source of claim 1, wherein itis provided with a number of anodes having a shape approximating to ahemispherical one and located on a substantially planar base or madetherein, and the cathodes are spire-shaped, said spires being disposedabove said anodes essentially at the center thereof.